▎ 摘　　要

The paper reports the synthesis of pentaethylenehexamine and histidine-functionalized graphene quantum dots (PEHA-GQD-His). The as-synthesized PEHA-GQD-His was designed as the fluorescence probe for microRNA fluorescence biosensing nanoplatform coupled with molecular beacon double cycle amplification strategy. In the strategy the target microRNA specifically binds the molecular beacon to trigger the target cycle and the molecular beacon cycle, leading to an efficient DNA nanoassembly process on the PEHA-GQD-His surface. The resulted G-quadruplexes properly fold and bind hemin to produce stable G-quadruplex/hemin complexes. Then, the fluorescence of PEHA-GQD-His is significantly quenched via the photoinduced electron transfer by the hemin attached on the PEHA-GQD-His and the electron acceptor of O-2, which is in situ generated by the decomposition of H2O2 under the catalysis of G-quadruplex/hemin DNAzymes. The study indicates that the design of target and beacon double cycle creates highly specific and efficient DNA nanoassembly on the PEHA-GQD-His surface. The DNAzyme-modulated double quenching mechanism achieves to high fluorescence quenching efficiency of PEHA-GQD-His. The introduction of pentaethylenehexamine increases the fluorescence emission intensity of PEHA-GQD-His. The introduction of histidine in PEHA-GQD-His remarkably enhances the catalytic activity of G-quadruplex/hemin DNAzymes towards H2O2. As a result, the proposed biosensing nanoplatform exhibits an ultrasensitive fluorescence response towards microRNA. Its fluorescence linearly reduces with increasing microRNA-141 concentration in the range of 1.0 x 10(-18)-1.0 x 10(-12) M with the detection limit of 4.3 x 10(-19) M. The analytical method has been successfully applied to fluorescence detection of microRNA-141 in human serum.